This relates generally to image sensors, and more specifically, to backside illumination (BSI) image sensors.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include photodiodes and other operational circuitry such as transistors formed in a front surface of the substrate. A dielectric stack is formed on the front surface of the substrate directly on top of the photodiodes. The dielectric stack includes metal routing lines and metal vias formed in dielectric material. Light guides are often formed in the dielectric stack to guide the trajectory of incoming light.
A color filter array is formed over the dielectric stack to provide each pixel with sensitivity to a certain range of wavelengths. Microlenses may be formed over the color filter array. Light enters from a front side of the image sensor (i.e., light enters the microlenses and travels through the color filters into the dielectric stack). An image sensor used in this way is referred to as a front-side illumination (FSI) image sensor.
The light guides are used to direct the incoming light towards the photodiodes. The incoming light, however, is sometimes reflected and absorbed by the metal routing lines and vias when traversing the dielectric stack. This results in undesirable pixel crosstalk and signal degradation.
To address these issues, backside illumination image sensors have been developed. These sensors are, however, also subject to crosstalk as stray light that is intended for a given photodiode can still inadvertently leak into regions of the substrate that can potentially corrupt the signal integrity of the image sensor. It is within this context that the embodiments described herein arise.